Resistance of rice to blast fungi is classified into two types: true resistance and field resistance (Non-patent Document 1). The former is based on hypersensitive reactions, and is a very effective and qualitative resistance highly specific to race. It has been known by experience that a variety introduced with a single resistance gene loses its effect in a few years due to the appearance of fungi compatible with the gene. On the other hand, field resistance is defined as the difference of resistance among varieties that is observed under conditions where true resistance is not functioning. Although the effect of field resistance is smaller compared to true resistance, it is practically useful because it has low race specificity and can confer continuous resistance to varieties.
Thirty or more kinds of genes associated with true resistance are known, and of these, Pib and Pita genes have been isolated (Non-patent Document 2). It has been found that these genes are NBS-LRR class genes having nucleotide binding sites (NBS) and leucine-rich repeats (LRRs), with structures similar to previously reported plant disease resistance genes. Like other disease resistance genes, products of plant resistance genes are considered to have a receptor function, directly or indirectly recognizing products of nonpathogenic genes in pathogens corresponding to the diseases. It has been actually revealed that Pita physically and directly binds to a nonpathogenic gene product.
As for field resistance, Japanese upland rice varieties are known to have excellent traits, and chromosomal positions of multiple gene loci involved in field resistance have been identified (Non-patent Document 3). However, the structure and expression mechanisms of the genes have not been elucidated, and thus field resistance cannot yet be efficiently used for breeding selection compared to true resistance. Multiple chromosome regions in the West Africa upland rice variety Moroberekan have been reported to play a role in incomplete resistance, which is a concept similar to field resistance (Non-patent Document 4); however, no genes have been identified.    [Patent Document 1] Japanese Patent Application Kokai Publication No. (JP-A) 2000-93028 (unexamined, published Japanese patent application)    [Patent Document 2] JP-A 2000-342262    [Patent Document 3] (Granted/Registered) Japanese Patent No. 3376453 (P3376453)    [Patent Document 4] JP-A 2003-88379 (P2003-88379A)    [Patent Document 5] JP-A 2003-199577 (P2003-199577A)    [Patent Document 6] JP-A 2003-199448 (P2003-199448A)    [Patent Document 7] JP-A 2004-329215 (P2004-329215A)    [Non-patent Document 1] Rice Blast and Breeding for its resistance. Kousaka and Yamazaki eds., p 175-186, 1980, Hakuyusha    [Non-patent Document 2] Wang et al., Plant J 19:55-64, 1999; Bryan et al. Plant Cell. 12: 2033-46, 2000    [Non-patent Document 3] Fukuoka and Okuno, Theor Appl Genet. 03:185-190, 2001    [Non-patent Document 4] Wang et al., Genet 136:1421-1434, 1994